1. Field of the Invention
The present invention generally relates to plunger lift technology and, more particular, is concerned with a casing differential pressure based control method for gas-producing wells.
2. Description of the Prior Art
In a typical plunger lift system, such as seen in FIG. 1, a gas-producing well W employs a freely movable plunger P disposed within a tubing T in the well that is capable of traveling vertically in the tubing T as the well W is cycled between shut-in and open conditions. The well W is shut-in for an interval during which the pressure of gas G gradually elevates within the well casing C. When the pressure of gas G reaches a desired level, a master gas flow control valve A, commonly referred to in the industry as the A valve, is opened causing the plunger P to be propelled by the accumulated gas pressure from a lower initial position, at a bottom bumper B, upward in the tubing T toward an upper terminal position adjacent to a plunger arrival sensor S at the wellhead. Fluid and gas above the plunger P in the tubing T discharges from the wellhead through a horizontal conduit H into a flow line L, called a gas sales line, leading to a separator (not shown). At the separator, gas and water separate from one another and are routed to separate storage vessels. The plunger P is held at the upper terminal position until the gas pressure diminishes to an extent permitting the plunger P to fall under gravity to its lower initial position.
Many plunger lift systems, in addition to the master flow control or A valve, will typically utilize a second flow control valve, commonly referred to in the industry as the B valve (FIG. 2), with an electronic controller E to control cycling of the well between shut-in and open times and thereby the production of gas from the well. As mentioned above, the A valve is interposed in the gas sales line L whereas the B valve is interposed in a vent line (not shown) that leads to a containment tank or pit or sometimes directly to atmosphere. The gas sales line L is under a higher pressure than the vent line. The shut-in and open times of the cycles providing optimum well production will vary from well to well due to the differing conditions of the wells.
The electronic controller E is programmed by an operator to set close, open, delay and shut-in times for the A and B valves so as to control the times of opening and closing of the A and B valves as well as other functions to provide for desired production and sales of gas from a given well. Also, the plunger lift system typically employs the arrival sensor S at the wellhead to sense the arrival of the plunger P at the upper terminal position. The arrival sensor S sends an electrical signal to the controller E in response to the arrival of the plunger P.
The employment of the B valve is necessary on many wells due to pressure fluctuations experienced in the high pressure gas sales line L of such wells which can impede efficient production of gas from the well W. There are various causes of pressure variation, the main ones being conditions created by mechanical equipment attached to the gas sales line L or the weather. When gas sales line pressure fluctuates enough that it becomes too great for the well casing pressure to exceed it and drive the plunger P to the upper terminal position of the wellhead, the plunger P may stall before reaching the surface or not arrive at the upper terminal position within the preset open time of the A valve. The controller E is programmed to then close the A valve and open the B valve to vent the well casing C to atmosphere or a low pressure tank or pit and thereby permit the plunger P to reach the upper terminal position and blow out the fluid that has accumulated above the plunger P. After the plunger P arrives and blows out the fluid, the controller E will shut the B valve and open the A valve and thus commence sale of gas from the well W through the A valve and the gas sales line L.
The key to efficient gas production is to prevent a head of fluid from building in the tubing T above the plunger P that will exceed the gas pressure in the casing C and prevent lifting the plunger P and fluid to the wellhead. To keep the well casing C and tubing T relatively free of fluid, the plunger P must be cycled at a rate generally matched to the rate that fluid comes into the well casing C from the production formation through perforations in the casing D so as to allow gas to come into the well casing C through the same perforations. The function times programmed in the electronic controller E by the operator are selected based on the particular condition of the well. As the well ages there is typically less gas pressure and more fluid flowing into the well casing C. An operator, therefore, needs to periodically monitor the operation of the well and change the programmed function times as the condition of the well changes.
Electronic controllers have been devised in the past to relieve an operator of this task by automatically counting the number and times of past plunger trip times, comparing them with target numbers and times and changing the programmed times using an algorithm stored in the memory of the electronic controller. While automatic controllers have accomplished this task in a generally satisfactory manner, still they are complicated and expensive and generally fail to optimize the A valve open time when gas produced by a well is being sold. Typically, these controllers will be programmed to close the A valve and terminate gas sales much earlier than needed resulting in a substantial reduction in the level of sales.
Consequently, a need exists for improvement of control of A valve open time to improve the cycling of a gas-producing well between shut-in and open times and thereby improve the efficiency of gas production and sales from the well.